Rate constants and the H atom branching ratio of the reactions of the methylidyne CH(X2Π) radical with C2H2, C2H4, C3H4 (methylacetylene and allene), C3H6 (propene) and C4H8 (trans-butene)

Abstract

The reactions of the CH radical with several unsaturated hydrocarbons C₂H₂ (acetylene), C₂H₄ (ethylene), C₃H₄ (methyl-acetylene and allene), C₃H₆ (propene) and C₄H₈ (trans-butene) were studied at room temperature, in a low-pressure fast-flow reactor. CH(X²Π, v = 0) radicals were obtained from the reaction of CHBr₃ with potassium atoms. The overall rate constants at 300 K are CH + C₂H₂: (3.6 ± 0.6) × 10⁻¹⁰, CH + C₂H₄: (3.1 ± 0.6) × 10⁻¹⁰, CH + C₃H₄ (methyl-acetylene): (3.4 ± 0.6) × 10⁻¹⁰, CH + C₃H₄ (allene): (3.6 ± 0.6) × 10⁻¹⁰, CH + C₃H₆ (propene): (4.2 ± 0.8) × 10⁻¹⁰ and CH + C₄H₈ (trans-butene): (4.0 ± 0.80) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ (errors are cited at the level of ±1σ). Absolute atomic hydrogen production was determined by vacuum ultra-violet (VUV) resonance fluorescence, H production from the CH + CH₄ reaction being used as a reference. Observed H branching ratios for these CH reactions were: C₂H₂: 0.90 ± 0.08, C₂H₄: 0.94 ± 0.08, C₃H₄ (methyl-acetylene): 0.98 ± 0.08, C₃H₄ (allene): 0.97 ± 0.08, C₃H₆ (propene): 0.78 ± 0.10, C₄H₈ (trans-butene): 0.69 ± 0.12 (errors are cited at the level of ±1σ). A compilation of the available kinetic data on these reactions has been made in order to propose rate coefficients for each possible channel of the different reactions for astrochemical models.